Communications networks Operations, Administration, and Maintenance (OAM) protocols are used to monitor the connectivity and performance parameters, such as propagation delay or information loss, of communications paths between communications nodes in the networks. For packet switched networks, these protocols may entail transmission and reception of special packets, known as OAM packets, which conform to standardized protocols. For networks based on Ethernet, standardized OAM protocols have been defined by the ITU-T in Recommendation Y.1731 and by the IEEE in both 802.1ag (CFM) and 802.3ah (EFM) standards. For networks based on the Internet Protocol (IP), the IETF has specified Bidirectional Forwarding Detection (BFD) to monitor basic connectivity, the One-Way Active Measurement Protocol (OWAMP), and the Two-Way Active Measurement Protocol (TWAMP) to monitor performance parameters. For networks based on MPLS, the IETF has defined OAM protocols in RFCs 6374, 6375, 6426, 6427, 6428, and 6435.
Some OAM protocols for packet switched networks are one-way “heartbeat” protocols, in which a first communications node sends Continuity Check or “CC packets” to a second communications node, usually at some constant rate of packets per second. If the second communications node does not receive packets for some preconfigured period, conventionally chosen to be somewhat more than the time required for three CC packets to arrive, it declares a continuity fault. If most CC packets sent by the first communications node are received by the second communications node, the percentage of packets lost may be counted in order to determine the Packet Loss Ratio (PLR). In addition, if the first and second communications nodes share a common clock, or have synchronized clocks, one-way protocols may be used to measure the propagation delay across the communications path between them. Even without a common clock, one-way protocols may be used to measure Packet Delay Variation (PDV).
Some, OAM protocols are two-way “loopback” protocols, in which a first communications node sends loopback or “LB packets” to a second communications node, which reflects these packets back to the first communications node. Loopback protocols may be used to detect loss of continuity in either direction, measure PLR and PDV, and to measure round-trip delay without the need for a shared common clock or synchronized clocks.
In order to perform OAM functionality, the first communications node may periodically create, configure, and transmit to the second communications node a sequence of OAM (either CC or LB) packets. OAM packets may be transmitted at a low rate of for example, one per second, or at higher rates of tens or hundreds of packets per second. For example, if it is advantageous to detect a loss of continuity within 30 milliseconds, and the condition for detection is the conventional three lost packets, then OAM packets must be sent no less frequently then every 10 milliseconds, i.e., at least 100 packets per second. The first communications node may need to participate in a large number of such “OAM sessions”, differing in packet characteristics (e.g., priority marking, packet size). Thus the computational toll of OAM protocols on the first communications node may be significant.
If it is desired to measure round-trip delay with a loopback protocol, the loopback packet must be time-stamped by the first communications node with a transmission time, TT(1). Upon receiving the loopback packet, the second communications node time-stamps the received loopback packet with the reception time, TR(2). Once the loopback response packet has been properly formed and is ready to be transmitted back to the first communications node, the second communications node time-stamps the response packet with the transmission time, TT(2) and transmits the response packet to the first communications node. Upon reception by the first communications node, the first communications node notes the arrival time with a fourth time-stamp TR(1). The round-trip delay is given by the total transit time TR(1)−TT(1) reduced by the dwell time in the second communications node TT(2)−TR(2).
Whereas it is possible to measure round-trip delay using a conventional OAM protocol it is not possible to directly measure a one-way delay in either direction via conventional loopback OAM. If it is known that the path between the first communications node and the second communications node consists of a single physical link, or this path is guaranteed to traverse the same network elements and the loading on all of these network elements is similar, then half the round-trip delay can be taken as a rough estimate of the one-way delay. In other cases there is no way of deducing the one-way delay based on conventional loopback OAM alone.